Image intensifiers are inherently color-blind. The cathode material initially produces only one electron per photon event, no matter what the color of the photon, if the photon is within the range of spectral sensitivity. Therefore, the number of electrons and the electron energies are amplified without regard to the color of the original photons. The image intensifier (I.sup.2) output is a direct view of a monochromatic (usually green) phosphor, or a monochrome television signal using a CCD camera either coupled to the phosphor or driven by the amplified electrons (EB-CCD).
There is a need for color image intensifiers, both for direct view and for electronic output. Several applications may require color image intensification, such as when the illumination contrast is poor, but the color contrast is good, when color is required for object identification, or when a color format is preferred (as in TV news reporting). The following are examples of instances when color image intensification is desirable.
Imaging for surgery (where too much heat and light can dry out tissue)
For microscopes used on light-sensitive samples;
Intrusive medical imaging;
Low-light color video camera (for law-enforcement, television, or consumer markets)
Infrared image conversion (for real-time natural resource or pollution surveying); and
Night surveillance cameras.
This invention disclosure details a manufacturing process for electronic output color image intensifiers (or light-intensified color TV cameras).
A common method of generating a color video signal from a CCD camera is to apply narrow stripe color filters directly to the CCD image plane, so that adjacent charge packets are associated with different colors. Charge packets from different colors are sorted and recombined electronically into a color video signal. The same scheme can be used to make a color I.sup.2 camera, as described in "Low Light Level Color Camera with One CCD", SPIE Electron Image Tubes and Image Intensifiers, 1990. The same type of CCD is used, except the narrow stripe color filters are left off the CCD. The filters are instead applied to the input image plane of the I.sup.2 parent device, and the CCD is coupled to the I.sup.2 output. The input stripe filters can be located at a secondary image plane, applied to a fiber optic faceplate, or sandwiched between the faceplate and photocathode. The CCD can be coupled to the I.sup.2 phosphor output using fiber optics, or could be installed in the I.sup.2 device for direct electron bombardment (EB-CCD).
There are several difficulties with this method, viz., it is very difficult to get good alignment and registration between the CCD rows or columns at the I.sup.2 output with the stripe filters at the I.sup.2 input. The filter set must have precise stripe spacing and angle with respect to the CCD. Any distortion in the tube or coupler results in bad alignment and registration between the filter stripes and CCD columns, which produces alias patterns and bad color. Further, the CCD pixel size must be matched to the parent I.sup.2 device resolution. Typical CCD cameras have pixels (and filter stripes) which are narrower than the resolution of typical I.sup.2 tubes. As a result, light coming through one filter stripe causes in an output signal on several adjacent columns of the CCD.
The first problem, relating to alignment and registration, can be overcome by having a precisely-dimensioned filter set which can be positioned and adjusted in front of the input while the device is operating. The most practical method is to use a fiber optic faceplate so that the image plane is accessible, and the filter set is external to the tube. Alternately, the precisely-dimensioned filter set can be fixed on the image plane (possibly sandwiched between the faceplate and cathode), and the CCD can be coupled to the output by a distortion-free fiber optic, so that the final CCD position can be adjusted while the system is operating. These methods are discussed in "Low Light Level Color Camera with One CCD", SPIE Electron Image Tubes and Image Intensifiers, 1990.
The last problem relating to resolution of the I.sup.2 can be overcome by using a demagnifying invertor I.sup.2 tube or an I.sup.2 tube with a demagnifying fiber optic taper at the output. This allows a small CCD to be matched to the full area of a larger I.sup.2 device with larger filter stripes. Unfortunately, this makes the first two problems much worse due to image distortions. Distortions prevent an equally-spaced straight-line stripe filter set from aligning with the CCD. Distortions require an inversely distortion-matched filter set.
It is, therefore, an object of the present invention to provide a color preserving I.sup.2 utilizing input filters which are precisely aligned with corresponding CCD pixels receiving the I.sup.2 output signal.